How are the tubes sized up? (outer diameter)
Each tube description provides outer tube diameter. In addition, another value provided for a given dimension (e.g. 42x2, 42x3) determines the thickness of the outer wall of the tube. The tube diameter reflects the outer diameter. The associated tubefittings have the same tube diameter in the product name, therefore you always know whether you have the correct size of a connector for your tube. The inner diameter of the fittings is slightly bigger, so you can easily slide them over the tubes.